1. Field of the Invention
The present invention relates to a zoom lens system suitable for use in a photographing optical system for, for example, a silver-halide film camera or a digital still camera.
2. Related Background Art
Up to now, there has been known a so-called negative-lead type zoom lens including the preceding lens unit having a negative refractive power. The negative-lead type zoom lens has advantages in that    (a) a closest focusing distance is relatively short,    (b) a field angle is relatively easy to widen, and    (c) a back focus is relatively easy to lengthen.Therefore, the negative-lead type zoom lens has been adopted to a wide-angle zoom lens in many cases.
Zoom lenses have been proposed in, for example, JP S57-011315 A (corresponding to GB 2080966A), JP S58-095315 A (corresponding to U.S. Pat. No. 4,591,235 B), JP S59-229517 A, JP S60-055313 A, JP S60-087312 A (corresponding to U.S. Pat. No. 4,653,873 B), JP S61-062013 A, JP S61-123811 A, JP S62-063909 A, JP H02-136812 A, JP H04-235515 A (corresponding to U.S. Pat. No. 5,329,401 B), JP H04-163415 A (corresponding to U.S. Pat. No. 5,132,848 B), JP H05-019170 A (corresponding to U.S. Pat. No. 5,264,965 B), JP H05-313065 A, JP H06-082698 A, JP H07-287168 A (corresponding to U.S. Pat. No. 5,585,970 B), JP H10-082954 A, JP 2000-338397 A (corresponding to U.S. Pat. No. 6,710,931 B), JP 2002-287031 A, and JP 2629904 B. Each of the zoom lenses includes a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power, which are disposed in order from an object side, and zooming is performed by moving at least two lens units of those lens units.
In recent years, there has been a demand for a zoom lens with wide field angle and a highly improved resolution for a single-lens reflex digital camera.
In general, when the refractive power of each of the lens units in the zoom lens is increased, a distance necessary for each of the lens units to move in order to obtain a predetermined zoom ratio is reduced. Therefore, it is possible to widen the field angle while the entire lens length is shortened.
However, merely increasing the refractive power of each of the lens units leads to large variations of aberrations caused by zooming. In particular, when a wide-field angle is to be realized, it is hard to obtain preferable optical performance over the entire zoom range.
When the filed angle is to be widened, it is hard to ensure the back focus or to correct distortion and astigmatism. Therefore, high performance is not obtained or a size of the zoom lens is likely to increase.